Capacitive touch sensors are used as user interfaces to electronic equipment, e.g., computers, mobile phones, personal portable media players, calculators, telephones, cash registers, gasoline pumps, etc. In some applications, opaque touch sensors provide soft key functionality. In other applications, transparent touch sensors overlay a display to allow the user to interact, via touch, with objects on the display. Such objects may be in the form of soft keys, menus, and other objects on the display. The capacitive touch sensors or touch screens are activated (controls a signal indicating activation) by a change in capacitance of the capacitive touch sensor when an object, e.g., a user's fingertip, causes the capacitance thereof to change.
Such touchscreens and touch sensors are highly affected by their environment. Consequently, an important characteristic of a capacitive touch solution is its ability to reject noise. Other sensor applications may suffer under similar restrictions.
In particular, capacitive sensor applications are tested under IEC (International Electrotechnical Commission) Standard 61000-4-6 for conducted noise immunity. This type of noise appears on the sensors' signals as random variations off an average and gets worse when the user's body is near the sensor.
With noise present on the signal, the sensor must be sampled multiple times to verify the result was not an accident due to random variation. As the amount of noise increases, the amount of additional samples must also increase. This slows the overall response time of the system, resulting in an undesirable user experience. Scanning at one, fixed sampling rate will cause the system to perform well at some noise frequencies, while other frequencies may cause the signal to degrade to the point of inoperability.